Swelling varieties of coals mined from the midwestern area of the United States have a strong tendency to expand or bloat or exude tarry liquids when heated during carbonizing or pyrolyzing operations. This ordinarily takes place when heating coals to about the 600.degree. F. to 1200.degree. F. range while volatile matter becomes evolved and the coal matrix becomes softened. These midwest coals generally have a free-swelling index ranging from about 3 to 8 and have strong to mild coking or caking properties. The bloating characteristic tends to cause bedded and fired coals to become impervious during carbonizing or pyrolyzing reactions, and the ultimate charring of the tarry surface constituents causes agglomeration or porous beds into impervious masses or cakes, which inhibit draft-solid reactions. Such phenomena and reactions are carried out when coal is subjected to combustion, gasification, carbonization, or pyrolysis, using a heated gaseous media to react with the coal particles.
Prior art techniques for inhibiting coalescence of the coal bed involve some of the following approaches:
1. Coarse-sized coal particles are used with large interparticle voids to permit minor swelling to take place without complete obstruction or sealing of voids.
2. Inert recycle materials, such as previously fired coal (coke), are mixed with the coal as a partial of the bed.
3. Mechanical bed agitators or bed mixers are employed for imparting permeability while the bed is becoming fired.
Another method of inhibiting bed coalescence, to a certain extent, involves the use of a pelletizing operation. Coal is ground to about 65 mesh and is mixed with water and balled into discrete, close-sized green pellets. Intersticial voids of the pellet may occupy about 20 percent of the pellet volume; hence, these voids can allow for a small amount of bloating without pellet expansion during firing. Also, voids between individual pellets can allow a certain amount of bloating before the mass becomes impervious. This is similar to the use of coarse-sized coal for bed firing reactions.
The use of the pelletizing process for production of carbonized or pyrolyzed pellets is described in U.S. Pat. No. 4,111,755, the subject matter of which is incorporated herein by reference. Generally, that patent relates to a fixed sulfur fuel which is a highly upgraded material with many beneficial aspects with respect to its use as a source of energy from combustion or as a reagent for gasification. The fuel is pelletized coal, or pellet coke, and is produced by pyrolyzing balled mixtures of fine coal with limestone and/or alkaline oxides at high temperatures within a reducing or slightly oxidizing environment to cause simultaneous high temperature decomposition of the hydrocarbonaceous matter of the coal and calcination with sulfur fixation of the basic constituents.
More specifically, sulfur-bearing coal and limestone are proportioned in amounts which will cause the calcium in the limestone to react with a large amount of sulfur in the coal. The proportioned mixture is ground and blended, and then balled or compacted to form pellets. Those pellets are then subjected to either a carbonizing or pyrolyzing technique at a temperature of at least 800.degree. F., and preferably between 1200.degree. F. and 2200.degree. F. These techniques are carried out on a traveling grate machine. If a pyrolyzing technique is employed, the firing operation is carried out in a reducing atmosphere, and preferably on a sealed, circular traveling grate machine of the type shown in U.S. Pat. No. 3,302,936. If a carbonization technique is employed, air is admitted to the firing zone and the firing operation need not be conducted in a sealed atmosphere. In either case, however, the presence of large amounts of carbon ensures a reducing condition in the traveling bed. During the firing operation, the limestone is calcined and the sulfur is fixed in a calcium compound which becomes stabilized in the ash after the pellet is burned as a fuel. The use of very high swelling coals in this process causes certain coalescence problems in firing the pellets by traveling grate operations, and these are not completely overcome by direct pelletizing operations.
When firing green pellets containing troublesome, high swelling and low softening coals, it is noted that:
1. The individual particles of the pellet bloat extensively to cause the pellet to expand and exude tarry liquid matter on the pellet surfaces.
2. Inter-pellet expansion and final adhesion from sticky surfaces occur between the soft expanding pellets which can practically fill the interpellet voids.
3. On some occasions, the swelling is followed by shrinking during firing, similar to tarry or foamy bubbles which collapse when they lose gas. This shrinking phenomenon causes large bed cracks within a partially coalesced bed because individual pellets of the coherent mass shrink and cause the bed to crack.
4. Large cracks within coalesced beds are avenues of short-circuiting of draft, and this causes inefficiencies of bed firing operations.